The invention relates generally to electrical testing of electronic cards such as bare die laminate ball grid arrays and deals more particularly with an apparatus for pressing contacts of an electronic card against contacts of a test socket to ensure proper electrical connection during a test.
It was known to mount an integrated circuit or "chip" to one face of a single or multi-layer printed circuit board by solder balls such that contacts of the chip face the substrate ("flip-chip"). Typically the board and chip are both rectangular, and the chip has a smaller surface area. Then, an array of surface pads are formed by photo-lithography of copper on the other face of the board. Next, plated through holes (PTHs) are formed in the card to interconnect the chip to the array of pads. Next, solder balls are formed on the pads by a reflow process such that this face of the board contains an array of co-planar solder balls. By way of example, the solder balls comprise a tin/lead composition and have a diameter of 25-40 mils. The resultant card is called a "bare die laminate ball grid array" (BDL-BGA).
The next step is to test the electrical performance of the BDL-BGA. A "clam-shell" test structure was previously known to quickly interconnect the BDL-BGA to a test circuit. A base of the clam-shell test structure contains a test socket with an array of contacts. The BDL-BGA is placed in the socket such that the ball grid array rests over respective contacts within the socket. These contacts are the heads of pins. The pins are held in respective holes in the base of the test structure, and bottoms of the pins are soldered into a printed circuit board of electronic test equipment. The test structure also includes a top flap which is pivotally mounted to the base and includes a generally rectangular lip which protrudes downwardly toward the socket. (In a similar test structure, there is a plate sandwiched between the top flap and the base, the plate is pivotally mounted to the base and the plate includes a rectangular lip which protrudes downwardly toward the socket.) To prepare for the test, the top flap is closed such that the rectangular lip presses against the perimeter of the exposed surface of the chip to press the array of solder balls against respective contacts in the socket. Typically, each solder ball in the array requires 2-4 ounces of pressure and there can be hundreds or solder balls in the array. Therefore, considerable force is required to ensure proper electrical contact between the solder balls and the array. The chip typically comprises a hard silicon material and is therefore, brittle. Also, there may be internal stresses created by thermal expansion during formation of the BDL-BGA. The internal stresses are such that the perimeter of the upper surface of the chip is relatively sensitive to downward force. Consequently, the downward pressure of the test structure may break the chip, particularly in this case, where the rectangular lip presses against the perimeter of the surface of the chip.
Other lip shapes were also known. For example, U.S. Pat. No. 5,528,466 discloses a circular lip on the underside of a plunger. The plunger is pressed downwardly against the upper surface of an electrical component such as a surface mounted chip package or carrier. The pressure forces ball leads on the opposite surface of the electrical component against conductors on a printed circuit board. The illustrated plunger arrangement appears to contact a more central area of the electrical component than the foregoing prior art.
U.S. Pat. No. 4,683,423 illustrates a post with a circular lip which presses against the surface of a rectangular chip, although the diameter of the lip is approximately equal to the width of the chip. Consequently, the circular lip exerts considerable force on two edge regions of the chip.
During some electrical tests, the chip dissipates considerable heat, and external cooling is required. The clam-shell test structure described above includes an opening in the top flap, a break in the rectangular lip and an opening in the back of the test structure above and behind the chip. These openings form a continuous duct which is open to the air at both ends and open to the chip in the middle. A fan forces air through the duct to facilitate cooling of the chip. The test structure of U.S. Pat. No. 5,528,466 includes an air space around the chip. The test structure of U.S. Pat. No. 4,683,423 includes an axial hole in the post (above the chip) and another axial hole in the middle of the test socket (below the chip). Both holes are open to the ambient air.
A general object of the present invention is to provide a test structure for pressing an electronic card into a test socket in such a manner as to as to minimize the chance of breaking the chip or other parts of the electronic card.
Another object of the present invention is to provide a test structure of the foregoing type which facilitates cooling of the chip.